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Offline Mark Hardy

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Re: Impact speed
« Reply #60 on: October 03, 2013, 09:01:18 am »
and here's some light hearted sh^t... :biggrin:

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Offline lecap

Re: Impact speed
« Reply #61 on: October 03, 2013, 09:25:21 am »
:laughing4:
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Offline Bensien

Re: Impact speed
« Reply #62 on: October 03, 2013, 10:02:54 am »
Incidentally, for those interested, and ignoring crumple zones in the car,

hitting a concrete barrier at 50km/h is equally damaging as hitting the barrier at 200 km/h.

The car comes to a halt in the same time at both speeds and the force exerted is the same.  ;)

That would only be true if t was zero. If the time for both were identical, the rate of deceleration (a) on the one travelling at 200kh/h would be 4 times greater than the one travelling at 50 and since f=m*a,  f would be 4 times greater as well.

We should also bear in mind that regardless of the formula, we are still measuring average rates of change. A modern vehicle has crumple zones, designed to control the rate of deceleration of the passenger cell. When the limit of the crumple zone is reached, the rate of deceleration will change dramatically, which explains why a difference of only a few kph can mean the difference between walking away from an accident of being killed.

In a proper test the vehicles are fitted with accelerometers and other instruments that can measure average as well instantaneous rates of change and peak values, which are more important. .
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Offline SlŠinte Mhaith

Re: Impact speed
« Reply #63 on: October 03, 2013, 01:57:49 pm »
Momentum on the other hand is a vector quantity, so the combined momentum of the 2 vehicles in the example will be zero.

Reminds me of accident reconstruction calcs. The momentum (vecotrs) before the accident and after must be equal or the sum thereof zero or something like that.





Amazing how accurately you can calculate the initial speed of vehicles in an accident with only the impact point and end positions known.
Whish I could get back into or more experience in accident reconstruction.
 

Offline SlŠinte Mhaith

Re: Impact speed
« Reply #64 on: October 03, 2013, 02:15:21 pm »

Two cars driving into a wall at 100km ph will disperse the same energy as two cars driving into each other.

In other words, in this instance, it doesn't matter whether you drive into a wall or head on into another car...

No I'm afraid not.....

I'm sort of getting tired of saying the same thing over and over, but if you drive into a stationary wall at 100 the relative speed is 100

If you have a head on with another car doing 100, the relative speed is 200.

Whether you like it or not, the laws of physics say it FOUR TIMES the kinetic energy all other things being equal.


I think Archangel is right.

You can't use relative speed for calculation of energy.  You calculate the energy of each vehicle and then add the two together.


Although the energy or momentum of the vehicles before the accident is important you also need to look at the dispersion of energy after the accident.
- A car driving into a wall: All the energy is absorbed in the car. The wall is not deformed at all.
- Same car driving into another stationary car.  The energy before the accident is the same as above but now the two cars share the absorption of the energy. They thus each have halve the damage.
-  Now double the relative speed and your energy double but both still share the deformation and thus equal to driving into a wall.
 

Offline J-dog

Re: Impact speed
« Reply #65 on: October 03, 2013, 02:48:47 pm »
Incidentally, for those interested, and ignoring crumple zones in the car,

hitting a concrete barrier at 50km/h is equally damaging as hitting the barrier at 200 km/h.

The car comes to a halt in the same time at both speeds and the force exerted is the same.  ;)

That would only be true if t was zero. If the time for both were identical, the rate of deceleration (a) on the one travelling at 200kh/h would be 4 times greater than the one travelling at 50 and since f=m*a,  f would be 4 times greater as well.

We should also bear in mind that regardless of the formula, we are still measuring average rates of change. A modern vehicle has crumple zones, designed to control the rate of deceleration of the passenger cell. When the limit of the crumple zone is reached, the rate of deceleration will change dramatically, which explains why a difference of only a few kph can mean the difference between walking away from an accident of being killed.

In a proper test the vehicles are fitted with accelerometers and other instruments that can measure average as well instantaneous rates of change and peak values, which are more important. .


I missed that. You are quite correct and my new choice of physicist  :thumleft:
 

Offline Bensien

Re: Impact speed
« Reply #66 on: October 03, 2013, 04:38:00 pm »
I donít have the right qualifications to be a physicist. Iím much to handsome and manly.
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Offline BlueBull2007

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Re: Impact speed
« Reply #67 on: October 03, 2013, 04:46:01 pm »
Actually its not physics, Bensein is our favorite mechanic. A Newtonian mechanic. ;D
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Offline Archangel

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Re: Impact speed
« Reply #68 on: October 10, 2013, 05:03:45 pm »
Kinetic energy can be calculated by the equation

E = 1/2 mV2

Assuming that the car weighs around 1 Ton or 1000 kg and travels at 100 km/h or 27.7 m/s, the equation will look like this:

  = 1/2 (1000kg)(27.7 m/s)2
  = 385 802 J
  or roughly 385 kJ

So each car is carrying 385 kJ of kinetic energy.

For you to bring that car to a stop, you will need to burn 385 kJ of kinetic energy by one of the following methods:
A. Be sensible and use your brakes (might take longer, but you'll survive)
B. Drive into a wall
C. Drive into an oncoming car of the same size and moving at the same velocity, but in the opposite direction

Either way, the car cannot disperse more energy than what it possessed in the first place.

If you have two cars, you will double the amount of energy coming into the equation, hence double the amount of energy dispersed if they collide head one, but exactly the same as two separate cars driving into walls.

However, if one car is driving double the speed into a wall:

E = 1/2 mV2
  = 1/2 (1000kg)(55.5 m/s)2
  = 1 540 125 J
  or roughly 1 540 kJ and is therefore 4 times as much as the car driving at 100 km/h

So to summarize:
Two cars into wall or two cars head on at 100 km/h disperses the same amount of energy, for there is only the kinetic energy that is brought into the equation by each moving car.

One car driving at 200 km/h has 4 times the kinetic energy of a car at 100 km/h and can therefore disperse 4 times as much energy, whether against a wall or another car...

Offline SlŠinte Mhaith

Re: Impact speed
« Reply #69 on: October 11, 2013, 09:25:15 am »
One car driving at 200 km/h has 4 times the kinetic energy of a car at 100 km/h and can therefore disperse 4 times as much energy, whether against a wall or another car...

Yes the damage to the speeding car when driving into an immovable wall will be 4 times more than one doing so at 100 km/h but when the same car speeding at 200km/h into a stationary car the damage to itself will only be double that of one driving at 100 km/h into a concrete wall...

 :peepwall:
 

Offline plaky

Re: Impact speed
« Reply #70 on: October 11, 2013, 10:45:43 am »
You oaks are too clever for me! If I hit a car travelling at 100km/h head on on my bike I am probably moer toe and all I will need is a Red Bull or a wing and a prayer!